YZ400 bogging big time, real hard to start

PumpkinHumper

You will need a head and base gasket. That is all I replaced. You may want to get the tensioner gasket also. You dont need a valve cover gasket unless yours is damaged or leaking or dosn't come off in 1 piece. Its rubber and rarley needs replacing. You may need a flywheel cover gasket. But mine came off fine. It may be an o-ring gasket. I cant remember. I just bought the yamaha gaskets. They wern't that bad. Then again I never looked aftermarket to see how bad the dealer rapped me.

Cometic may make a gasket kit I am not sure.

As for the flywheel puller. You yami shop should have an aftermarket puller for you. That is were I got mine. That puller fits a large % of yamaha MX bikes and quads.

Let me know were ya find the 1/4" torque wrench. No one local has one around here. I have found a few places online but nothing local. I need to get one also. have been putting it off.

PumpkinHumper

Hey just thought of something. There is a 1/2 moon clip that is under the cam cap that keeps the cam in alignment. When you take the cam cap off that clip has a tendancy to pop out and fall down into the bowels of the motor. Stuff rags into any and all holes in the head / cam chain tunnel before removing the cam caps. And then go slow so you dont drop that clip. The clip is right under the cap on the gear end of the cam, right on the big bearing.

yzf_rider400

Hey just thought of something. There is a 1/2 moon clip that is under the cam cap that keeps the cam in alignment. When you take the cam cap off that clip has a tendancy to pop out and fall down into the bowels of the motor. Stuff rags into any and all holes in the head / cam chain tunnel before removing the cam caps. And then go slow so you dont drop that clip. The clip is right under the cap on the gear end of the cam, right on the big bearing.

will do; what about the chain guides, do those get chewed up? The TT store has K&S and Cometic (I think) gasket kits for around $50 so I'm going to go the after market route.

PumpkinHumper

Cam chain guides should be fine. But you wont know till you get in there. If the chain is real stretched I could see it maybe chewing up a guide from slaping around or something. But they are probably fine.

Oh yea. If you decide to replace the tensioner they are like $75. I replaced mine but probably didn't need to. You decide. I dont think they go bad all that often.

bushy

The yzman will tell you different, but I was able to get replace the chain without taking off the head or cylinder. You have to unbolt the two guides and move them around while moving the chain and turning the crank over. Its easy with two people, you basicly yank and twist.

PumpkinHumper

Well as Bushy said there is 2 ways to do it. The correct way and the way Bushy and I both did it.

I see where you can unbolt the bottom mounts of one of the guides but it looks like the forward most one requires you to remove the head before it can be taken out.

When I was messing around with it; it seemed as though you could yank it past one of the chain guides but I was afraid to F#%K stuff up.

If you go the route without removing the cylinder head, how the hell do you get the new chain on? Is BASH the opposite of YANK?

You are correct. You can only loosten the rear chain guide. The front one is fixed. And yep pretty much yank/pry with screwdriver to get it past the front slide. Then bash pry to get it back in. Not exactly what the yamaha manual recomends. THe correct method is to remove just the head. That way you can get the guides outta the way. You decide.

And yes I did pry mine past the guides. Guity as charged.

Make sure that when you get the cams timed corectly to look at the lobes and see it they are as I had shown in the picts. There is a very slim chance that a cam sprocket has spun when it jumped. Its a slim chance but dont overlook it.

And remember to only use the front dot on the front cam and the rear dot on the rear cam. The rest of the dots on the cam gears dont matter other than the fact that they should be at 9:00, 12:00, and 3:00. The 2 outter dots are what you set the timing with by lining them up with the valve cover surface. That confused me a little untill I actually read the manual then it was clear as can be.

Are ya gonna put rings in it??? Common you are already there. Its like one more step and it is completly done. ya know ya wanna.

yzf_rider400

I took the cylinder head off today, making it super easy to get the timing chain off.

During the process of cleaning the carbon deposits off the piston I noticed a little pitting near the valve seats (closer to the center of the piston as opposed to around the outer edge). There were also a few small gouges/scratches near one of the exhaust valve seats.

Is the pitting normal? As for the scratches, do you think the previous owner may have done a poor job of scraping the piston (which would also mean the bike has been opened up before)?

Obviously I'm hoping that won't be a problem. I'd like to get her back together tomorrow. Thanks fellas!

PumpkinHumper

I took the cylinder head off today, making it super easy to get the timing chain off.

Told ya so

If you have questions about the head bring it to your dealer and let them take a look at it. The actual sealing of the valves is were my knowledge ends. Although I have heard of people putting some gas into the intake and exhaust ports. If it leaks past the valves you could have a problem.

yzf_rider400

I've got her back together (well almost). I still have to put the exhaust pipe, radiator and carb back on.

I opted not to put new rings in. My dad (a skilled mechanic) said that if I had good compression I probably did not need to; and it's more work. I did poor gas down the exhaust and intake ports and the valves are sealing just fine. My dad also thought the scratches weren't that big of a deal, just to be sure to get the burs off so they don't ignite.

Maybe tomorrow I'll get the radiator and stuff on, put some oil in 'er and fill her with coolant and have me a test ride. She better work now!!!

yzf_rider400

UPDATE: I'm still having some stuttering problems. I believe now that I did not jump cam-timing. Being a newbie I made a slight off TDC judgement of my cam lobes. I realized this once I put my new timing chain on and realized what a difference is made by just one tooth.

I'm actually glad I did the job though, when I pulled the old chain it was kinked and probably had never been replaced anyhow and I learned some things. I also checked the valves. All in spec, maybe on the tight side a little though.

yzman400, it might as well have been cam-timing and thanks for helping me get through the job. I feel better knowing now what I'm working with.

So I've come to the conclusion that it's electrical/ignition timing. I've gone through all the electrical checks, step by step. Everything is fine except for possibly my kill switch and my PRIMARY coil resistence.

primary coil resistence (should be .2 - .3 ohms) - my coil is varying between .3 - .8 ohms. My brothers 2001 250F has the same coil (part number) and his ohms vary between .1 - .3 ohms. At some point when I have some more time I'll switch them and see if that helps.

engine kill switch: (suppossed to show continuity when pressed). I'm getting eratic results here, half the time the switch gives me a reading the other half nothing. I disconnected it and the bike ran a lot better but started to stutter again (not as badly) after about 40 minutes of riding, compared to the five or so minutes with the kill switch connected.

Obviously I can't test the CDI unit, especially without a buddy who has the same bike. So after all this my question is: Do you think the above coil and kill switch readings are eradic or out of spec enough to give me a problem or should I suspect the CDI?

Thanks, this problem is sure taking me through all the twists and turns of getting a bike to run right.

PumpkinHumper

I would look at one more thing before you go dig too far into the electrical. What jetting is in the bike and what performance mods are done to the bike?

Is there a performance free flow pipe. Porting. High compression piston?

What jets are in the bike. What clip position is the needle on. Where is the fuel screw set. You said that it ran fine for 40 min before giving you trouble. Maybe its jetted rich and is killing the plug over time.

So we need to know what the folowing jets are:
main jet __
main air jet__
Pilot jet__
Pilot air jet__
Starter jet__
needle #__
Needle clip position__
Fuel screw position (turns out)__

Find this info and check against the manual to see what is supposed to be in there. If the bike is stock this should be pretty close to what should be in there. If the bike has been modified then the stock specs wont help ya much more than give ya a benchmark from where to start.

It could be a electrical part failing as it warms up. But that it rare. That kill switch is kinda puzzling. If you are getting spuratic readings might want to replace that. it is cheep.